1. Field of the Invention
This invention relates to a thin Fe-Ni alloy sheet for shadow mask having high press-working performance and method for manufacturing thereof and in particular to a thin Fe-Ni alloy sheet for shadow mask suitable for color cathode ray tube and method for manufacturing thereof.
2. Description of the Related Art
Recent up-grading trend of color television toward high definition TV has employed Fe-Ni alloy containing 34-38 wt. % of Ni as the alloy for shadow mask to cope with color-phase shift. Compared with low carbon steel which has long been used as a shadow mask material, conventional Fe-Ni alloy has a considerably low thermal expansion coefficient. Accordingly, a shadow mask made of conventional Fe-Ni alloy raises no problem of color-phase shift coming from the thermal expansion of shadow mask even when electron beam heats the shadow mask.
Common practice of making thin alloy sheet for a shadow mask includes the following steps. An alloy ingot is prepared by continuous casting process or ingot-making process. The alloy ingot is subjected to slabbing, hot-rolling, cold-rolling, and annealing to form a thin alloy sheet.
The alloy sheet is then processed usually in the following steps to form a shadow mask. Photo-etching forms passage-hole for electron beam on the thin alloy sheet for a shadow mask. The "passage-hole for electron beam" is hereinafter referred to as "hole". The thin alloy sheet for a shadow mask perforated by etching is hereinafter referred to as "flat mask". The flat mask is subjected to annealing. The annealed flat mask is pressed into a curved shape of cathode ray tube. The press-formed flat mask is assembled to a shadow mask which is then subjected to blackening treatment.
However, the shadow mask material of conventional Fe-Ni alloy has higher strength than conventional low carbon steel, which raises a problem of press-forming performance after perforation by etching. Softening is a means to solve the problem, where the crystal grain size is enlarged to a coarse one by conducting softening-annealing at 800.degree. C. or higher temperature. After the softening-annealing, warm-press is applied to carry out spheroidal forming. The temperature of 800.degree. C. is, however, in a high temperature region. Accordingly, from the viewpoint of work efficiency and economy, the development of manufacturing method to obtain such a low strength by a lower temperature softening-annealing has been awaited.
The prior art (A) is described in JP-A-H3-267320 (the term "JP-A-" referred to herein simplifies "unexamined Japanese patent publication"), where a method to decrease the strength of shadow mask material to a level preferred for press-forming is provided. According to the prior art (A), the recrystallization annealing is carried after cold-rolling. The temperature of recrystallization annealing is below 800.degree. C., and the embodiment of this invention adopts the operation at 730.degree. C. for 60 min. After the recrystallization annealing, the finish cold-rolling is conducted within a reduction ratio range of 5-20%. The prior art (A) produces a shadow mask having good press-forming performance giving 9.5 kgf/mm.sup.2 of proof stress at 200.degree. C.
Although the prior art (A) reduces the strength to a preferable level for press-forming by selecting the annealing condition of 730.degree. C. and 60 min., it does not satisfy the quality required to perform a favorable warm press-forming. Shadow masks prepared by the prior art (A) were found to gall the die and to generate cracks at the edge of shadow masks.
Nevertheless, cathode ray tube manufacturers try to carry out the annealing before press-forming at 730.degree. C. for 40 min. or shorter duration aiming to improve work efficiency and economy. In some cases, the annealing as short as 2 min. is applied. However, if such an annealing condition is applied to the prior art (A), the galling during press-forming becomes severe and the crack on shadow mask increases to raise serious quality problem.
The prior art (B) is introduced in JP-A-S64-52024 where a method to decrease intra-plane anisotropy, a mechanical property of material, is provided. In this method, at least two cycles of the cold-rolling and recrystallization annealing are repeated followed by the cold-rolling to increase hardness. A shadow mask base sheet having a low intra-plane anisotropy of elastic coefficient is obtained by selecting the reduction ratio of cold-rolling immediately before the final recrystallization within a range of 40-80%. When the base sheet is etched, annealed, and press-formed, it gives an excellent uniform deformation during press-forming resulting in a small deformation of etched-hole and free from irregular gloss and stringer defect.
According to the prior art (B), the intra-plane anisotropy is sufficiently small and the generation of penetration irregularity is at a low level, which raises no quality problem. Still, the prior art (B) induces cracks at the edge of shadow mask during press-forming.
Present color televisions require severer quality specification on color-phase shift because the color picture tubes direct to a brighter and more flat face than ever. The cathode ray tubes using the shadow masks prepared by the prior art (A) and the prior art (B) give partial color-phase shift under electron beam irradiation.